1. Field of the Invention
The present invention relates to a positive electrode for non-aqueous electrolyte lithium secondary batteries and a method of manufacturing the same.
2. Description of the Prior Art
Non-aqueous electrolyte secondary batteries having a lithium or lithium compound negative electrode have been studied extensively for their potentials for high voltage and high energy density.
Positive electrode-active materials of non-aqueous electrolyte secondary batteries proposed hitherto include oxides of transition metals and chalcogen compounds, such as Li.sub.x CoO.sub.2, Li.sub.x Mn.sub.2 O.sub.4, Li.sub.x FeO.sub.2, Li.sub.x NiO.sub.2, V.sub.2 O.sub.5, Cr.sub.2 O.sub.5, MnO.sub.2, TiS.sub.2, MoS.sub.2, and the like.
These active materials have layers or tunnel-like crystal structures allowing intercalation and deintercalation of lithium ions. Especially Li.sub.x CoO.sub.2 (hereinafter referred to as LiCoO.sub.2), Li.sub.x NiO.sub.2 (hereinafter referred to as LiNiO.sub.2), Li.sub.x MnO.sub.2 (hereinafter referred to as LiMnO.sub.2), and Li.sub.x Mn.sub.2 O.sub.4 (hereinafter referred to as LiMn.sub.2 O.sub.4) are noted as 4V-class positive electrode active materials for non-aqueous electrolyte lithium secondary batteries. LiCoO.sub.2 is a most promising positive electrode active material because of its favorable properties.
While cobalt contained in LiCoO.sub.2 is relatively expensive, LiNiO.sub.2, which is cheaper and thus allows stable supply, has been noted recently. LiNiO.sub.2 has a similar composition and structure to those of LiCoO.sub.2 and is expected to give a high capacity and a high voltage as a positive electrode active material for lithium secondary batteries. Improved synthesis has increased the capacity of LiNiO.sub.2 to the level exceeding LiCoO.sub.2. LiMn.sub.2 O.sub.4 has also excellent properties with a slightly lower capacity, and LiMnO.sub.2 has also been studied extensively for practical applications.
Binders generally applied for preparation of positive electrodes with these active materials are required to be chemically inactive and stable against organic electrolyte. Polytetrafluoroethylene having a structural formula represented by the formula (1), which is a chemically inactive and stable resin, is widely used as the binder. EQU (--CF.sub.2 --CF.sub.2 --).sub.n ( 1)
A typical method applied to preparation of electrodes includes the steps of mixing an aqueous dispersion of a resin powder with an active material, a conductive agent and other additives to prepare a paste, and applying the paste onto current collectors or electrically-conductive electrode supports to form electrodes. Another method comprises mixing of a powdery binder with an active material, a conductive agent and other additives in dry stage and pressure-molds the mixture.
When these complex oxides containing lithium and a transition metal are used as a positive electrode active material for preparation of positive electrodes of lithium secondary batteries, properties of the electrodes thus obtained are significantly affected by various conditions including the type of a binder applied and the type of a dispersion medium in which a mixture of an active material, a binder, a conductive agent and other additives are dispersed.
Electrodes containing LiNiO.sub.2 as an active material have varying properties according to the types of a binder and a dispersion medium. This is partly ascribed to the high reactivity of LiNiO.sub.2 with water. LiNiO.sub.2 easily causes an ion exchange reaction between protons and lithium ions in the presence of water, thus lowering the capacity. The former method using water as a dispersion medium undesirably lowers the capacity.
Preparation of electrodes in dry state, on the other hand, causes insufficient dispersion of constituents, thereby lowering the capacity.